In a coherent optical reception of differential M-ary phase-modulated signals, the incoming optical signal at some given frequency is non-linearly mixed (or superimposed) with a reference local oscillator that is set at a close-by frequency. The desired outcome is a mixing signal at the difference frequency, which carries the information (such as amplitude, phase, and frequency modulation) of the original higher frequency signal in the phase angle, but is oscillating at a lower, and hence more easily processed frequency.
The detected phase angle depends on two factors: the transmitted (sent) symbol and the frequency offset between the local oscillator laser and the center frequency of the incoming optical signal. In a homodyne coherent detection scheme, one strives to set the frequency of the local oscillator exactly at the center wavelength of the optical signal, and hence the frequency offset is zero. In practice, a perfect matching of the frequencies can hardly be achieved, and hence homodyne detection schemes are usually “intradyne” in practice, i.e., the wavelength of the local oscillator does not equal the center wavelength of the incoming signal, but lies very close to it and within the spectrum of the incoming signal. However, in many practical applications, heterodyne detection schemes, in which the wavelength of the local oscillator lies outside of the spectrum of the incoming signal are becoming increasingly popular, mostly because they can be implemented with a smaller number of optical components. In a heterodyne detection scheme, the frequency offset between the local oscillator laser and the center frequency of the incoming optical signal is electronically compensated by multiplying (“down-converting”) the received signal with a sine or cosine function with the difference frequency.
In both homodyne/intradyne and heterodyne detection, the outcome of the mixing are two baseband signals which contain an in-phase (I) component and a quadrature (Q) component of the signal. The in-phase component and the quadrature component can be represented grapically in a constellation diagram, and different range values (or bins) of the components can be associated with different signal bits. The assignment of quadrature components to the signal bits constitutes a decoding scheme that allows to extract the encoded signal from the in-phase component and quadrature component.
Due to imperfections in the electronic equipment and the limited frequency stability of the local oscillator laser, a perfect frequency offset can hardly be achieved in practice, neither in intradyne detection nor in heterodyne detection. As a result of an imperfect offset, the detected differential phase angles are rotated in the constellations. If the frequency offset is too large, the differential phase angles cannot be detected correctly any longer. For practical purposes, either the local oscillator frequency deviations or the down-conversion frequencies have to be kept below a threshold of about ±50 MHz to allow for reliable detection for the case of a 622 Mbaud (D) QPSK modulated signal. This is not too big a technical challenge, and has been shown to work fine and stable.
However, the above condition means that initially, when the system is started up and the local oscillator laser scans the available band for a signal, the scanning process needs to be conducted very slowly in order to get close enough to hit the right frequency window. A typical tunable laser covers an optical frequency band of about 4 Terahertz or more. Scanning that band with the resolution of 100 MHz requires 40,000 steps or more. After each laser frequency step, the digital signal processing decodes the received bits and tries to recognize the pre-determined frame delimiting bit pattern in the received data stream. For M=4, the bit stream consists of two bit pairs. However, it is generally unknown where the bytes themselves start within this bit stream. Typically, parallel frame hunter units are employed to scan the bit stream for the frame delimiting pattern, with each frame hunter employing a different offset within the bit stream. For M=4, four parallel frame hunter units are conventionally used. Assuming that a laser scans 100 steps per second, a full local oscillator laser scan can take up to 400 seconds, which is an unacceptably long time interval for many practical applications.
In the prior art, amplitude detection has been employed to increase the scanning speed. In these schemes, a coarse scan is conducted until a signal amplitude increase is detected, and only then the fine tuning of the laser is performed as described above. However, these schemes introduce additional complexities.
What is needed is an improved decoding method that allows to speed up the scanning process when the system is started.